An agricultural byproduct, barley straw, was chemically modified by a cationic surfactant, hexadecylpyridinium chloride monohydrate (CPC) and employed as an adsorbent to remove emulsified canola oil from aqueous solution. The textural and surface properties of the surfactant modified barley straw (SMBS) were characterized by N2 adsorption, FT-IR, SEM, surface acidic/basic groups and surfactant desorption. The low desorption of CPC from SMBS demonstrated a strong bonding of the CPC to straw surface. Several factors such as adsorption temperature, solution pH, loading of adsorbent, and particle size on oil adsorption were investigated. It was found that addition of CPC created a non-polar layer on barley straw surface thus endowing SMBS with much better adsorption capacity for oil removal from water. The adsorption was found less favorable at high acidic condition and the maximum adsorption capacity was observed at about neutrality. Larger particle size would result in lower adsorption while adsorption temperature would not affect oil adsorption significantly. The kinetic study revealed that equilibrium time was short and the isotherm study indicated that the oil adsorption was fitted well by the Langmuir model. The adsorption capacity determined from the Langmuir isotherm was 576.0 ± 0.3 mg g−1 at 25 °C.